The present invention is related to an optical element, and more particularly to an optical element capable of performing wavelength conversion and amplitude modulation simultaneously.
Quasi-phase-matched (QPM) nonlinear frequency conversion has been an attractive means for generating a wide range of laser wavelengths. For example, electric-field poled lithium niobate (PPLN) (L. E. Myers, G. D. Miller, E. C. Eckardt, M. M. Fejer, and R. L. Byer, Opt. Let. 20, 52 (1995)) has an effective nonlinear coefficient as high as 17 pm/volt, which makes PPLN an important QPM crystal for generating wavelength-tunable laser radiations. Other popular QPM crystals include periodically poled LiTaO3, KTiOPO4, RbTiOAsO4, and so on. Different QPM crystals have different material advantages. For example, when compared with PPLN, LiTaO3 has a better transparency in the shorter wavelengths, KTiOPO4 sustains higher laser fluence, and RbTiOAsO4 has a much lower coercive field for electrical poling. Quasi-phase-matched frequency conversion has been applied to numerous applications such as gas sensing, optical communication, and so on. In practice, signal modulation is desirable for sensitive detection or information encoding. In all available technologies, to the knowledge of one skilled in the art, frequency conversion and signal modulation have to be implemented separately.
The conventional amplitude-modulation techniques, to name a few, include the use of an optical chopper, the current-controlled driver for diode lasers, the Mach-Zehnder modulator, and the electro-optic birefringence crystal between two crossed polarizers. However, there is no effective amplitude-modulation technique during wavelength conversion without suffering from the disadvantages of the small modulation bandwidth, frequency chirping, or complexity associated with a conventional amplitude modulator.
In addition, Quasi-phase-matched nonlinear frequency conversion imposes a 180-degree reset on the relative phase among the three mixing waves every coherence length. In a QPM crystal, any phase error in a nonlinear domain affects the efficiency or the amplitude of the wavelength-converted signals at the downstream output. By manipulating the phase mismatch, it is possible to develop a novel device for simultaneous wavelength conversion and amplitude modulation in a monolithic QPM crystal.
It is therefore an object of the present invention to provide an effective amplitude-modulation technique during wavelength conversion without suffering from the disadvantages of the small modulation bandwidth, frequency chirping, or complexity associated with a conventional amplitude modulator.
It is further an object of the present invention to provide an optical element with simultaneous wavelength conversion and amplitude modulation in a monolithic QPM nonlinear optical crystal, by manipulating the phase mismatch to alter the amplitude of the wavelength-converted signals at the downstream output.
To achieve the aforementioned objects, an optical element capable of performing nonlinear frequency conversion and amplitude modulation simultaneously is provided. The optical element comprises a nonlinear optical crystal having an electrode-coated dispersion section in quasi-phase-matched (QPM) sections for electrically controlling the relative phase among the mixing waves therein by applying an electric field thereto, whereby performing the nonlinear frequency conversion and amplitude modulation simultaneously.
Preferably, the nonlinear optical crystal is the material capable of being made into quasi-phase-matched (QPM) nonlinear optical element. More preferably, the nonlinear optical crystal is made of the material selected from a group consisting of LiNbO3, LiTaO3, KTiOPO4, and RbTiOAsO4.
In accordance with one aspect of the present invention, the electrode-coated dispersion section is sandwiched between two quasi-phase-matched (QPM) sections.
In accordance another aspect of the present invention, the electrode-coated dispersion section is coated with metal electrodes on two opposite surface thereof.
Certainly, the nonlinear frequency conversion includes second harmonic generation (SHG), difference frequency generation (DFG), sum frequency generation (SFG), optical parametric generation (OPG), optical parametric amplification (OPA), and optical parametric oscillation (OPO).
In accordance with another aspect of the present invention, the electrode-coated dispersion section is sandwiched between quasi-phase-matched nonlinear gratings, the nonlinear gratings have both the grating vectors parallel to the wave vector of the mixing waves, and the amplitude modulation can be adjusted to either the linear or the nonlinear modulation regime with a direct-current voltage offset on the electrodes.
In accordance with another aspect of the present invention, the electrode-coated dispersion section is sandwiched between quasi-phase-matched nonlinear gratings. One of the nonlinear gratings has the grating vector parallel to the wave vector of the mixing waves, the other nonlinear grating has the grating vector forming an angle with respect to the wave vector of the mixing waves, and the amplitude modulation can be adjusted to either the linear or the nonlinear modulation regime by laterally translating the nonlinear crystal with respect to stationary mixing waves.
It is further an object of the present invention to provide a method for performing nonlinear frequency conversion and amplitude modulation simultaneously. The method includes the steps of fabricating a quasi-phase-matched (QPM) crystal with an embedded electrode-coated unpoled dispersion section, and applying an electric field to the electrode-coated unpoled dispersion section for controlling the relative phase among the mixing waves in the dispersion section, whereby performing the nonlinear frequency conversion and amplitude modulation simultaneously.
It is still an object of the present invention to provide an optical element capable of performing nonlinear frequency conversion and amplitude modulation simultaneously. The optical element includes a nonlinear optical crystal having multiple electrode-coated dispersion sections monolithically integrated in cascaded quasi-phase-matched (QPM) sections for electrically controlling the relative phase among the mixing waves therein by applying an electric field thereto, whereby performing the nonlinear frequency conversion and amplitude modulation simultaneously.
Preferably, each of the quasi-phase-matched (QPM) sections is the crystal section for performing one of the nonlinear optical processes, including second harmonic generation (SHG), difference frequency generation (DFG), sum frequency generation (SFG), optical parametric generation (OPG), optical parametric amplification (OPA), and optical parametric oscillation (OPO).
In accordance with one aspect of the present invention, the nonlinear optical crystal includes two electrode-coated dispersion sections interleaved in three quasi-phase-matched (QPM) sections for performing the nonlinear frequency conversion and amplitude modulation simultaneously.
It is further an object of the present invention to provide an optical element capable of performing nonlinear frequency conversion and amplitude modulation simultaneously. The optical element includes a nonlinear optical crystal having at least one electrode-coated dispersion section integrated in cascaded quasi-phase-matched (QPM) sections for electrically controlling the relative phase among the mixing waves therein by applying an electric field thereto, and a waveguide formed in the nonlinear optical crystal for guiding the mixing waves through the QPM sections and the dispersion section in the nonlinear optical crystal, whereby performing the nonlinear frequency conversion and amplitude modulation simultaneously.
In accordance with another aspect of the present invention, an optical waveguide, going through the QPM sections and the dispersion section of the present invention, can be fabricated on the surface of the nonlinear optical crystal and the conducting electrodes are coated with conducting materials on the two sides of the waveguide, whereby the relative phase of the mixing waves is controlled by the applied electric field on the electrodes.
It may best be understood through the following descriptions with reference to the accompanying drawings, in which: